linux-kernel - Re: [PATCH v2 1/2] arm: lib: xor-neon: remove unnecessary GCC

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Message-ID: <87mtzljz12.fsf@iwork.i-did-not-set--mail-host-address--so-tickle-me>
Date:   Fri, 13 Nov 2020 13:07:21 +0200
From:   Adrian Ratiu <adrian.ratiu@...labora.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Arvind Sankar <nivedita@...m.mit.edu>, kernel@...labora.com,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6
 warning

Hi Ard,

On Fri, 13 Nov 2020, Ard Biesheuvel <ardb@...nel.org> wrote:
> On Thu, 12 Nov 2020 at 22:23, Adrian Ratiu 
> <adrian.ratiu@...labora.com> wrote: 
>> 
>> From: Nathan Chancellor <natechancellor@...il.com> 
>> 
>> Drop warning because kernel now requires GCC >= v4.9 after 
>> commit 6ec4476ac825 ("Raise gcc version requirement to 4.9"). 
>> 
>> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com> 
>> Signed-off-by: Nathan Chancellor <natechancellor@...il.com> 
>> Signed-off-by: Adrian Ratiu <adrian.ratiu@...labora.com> 
> 
> Again, this does not do what it says on the tin. 
> 
> If you want to disable the pragma for Clang, call that out in 
> the commit log, and don't hide it under a GCC version change.

I am not doing anything for Clang in this series.

The option to auto-vectorize in Clang is enabled by default but 
doesn't work for some reason (likely to do with how it computes 
the cost model, so maybe not even a bug at all) and if we enable 
it explicitely (eg via a Clang specific pragma) we get some 
warnings we currently do not understand, so I am not changing the 
Clang behaviour at the recommendation of Nick.

So this is only for GCC as the "tin" says :) We can fix clang 
separately as the Clang bug has always been present and is 
unrelated.

> 
> Without the pragma, the generated code is the same as the 
> generic code, so it makes no sense to build xor-neon.ko at all, 
> right? 
>

Yes that is correct and that is the reason why in v1 I opted to 
not build xor-neon.ko for Clang anymore, but that got NACKed, so 
here I'm fixing the low hanging fruit: the very obvious & clear 
GCC problems.


>> ---
>>  arch/arm/lib/xor-neon.c | 9 +--------
>>  1 file changed, 1 insertion(+), 8 deletions(-)
>>
>> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
>> index b99dd8e1c93f..e1e76186ec23 100644
>> --- a/arch/arm/lib/xor-neon.c
>> +++ b/arch/arm/lib/xor-neon.c
>> @@ -19,15 +19,8 @@ MODULE_LICENSE("GPL");
>>   * -ftree-vectorize) to attempt to exploit implicit parallelism and emit
>>   * NEON instructions.
>>   */
>> -#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
>> +#ifdef CONFIG_CC_IS_GCC
>>  #pragma GCC optimize "tree-vectorize"
>> -#else
>> -/*
>> - * While older versions of GCC do not generate incorrect code, they fail to
>> - * recognize the parallel nature of these functions, and emit plain ARM code,
>> - * which is known to be slower than the optimized ARM code in asm-arm/xor.h.
>> - */
>> -#warning This code requires at least version 4.6 of GCC
>>  #endif
>>
>>  #pragma GCC diagnostic ignored "-Wunused-variable"
>> --
>> 2.29.2
>>